1. Field of the Invention
This invention relates to a mobile communication system which forms a micro-cell in the same area as the service area of a cellular system, and which selects, from the same frequency band that is allocated to the cellular system, channels that do not give rise to interference problems, and then shares their frequencies within the micro-cell.
2. Description of the Related Art
A mobile communication system which forms a micro-cell in the same area as the service area of an already constructed cellular system, and which shares the same frequency band, has been considered. Prior art examples of this sort are illustrated in FIG. 1 to FIG. 3.
FIG. 1 shows an arrangement of base stations, with an existing cellular system (hereinafter, termed "system A") forming service area 102 from a plurality of radio zones centering on base stations 101. The radio band allocated to this system is divided into a plurality of channel groups, and these are re-used in radio zones separated by a prescribed distance. Also deployed in service area 102 is base station 103 of a different system from system A, or of a system which, although the same as system A, can operate independently (hereinafter, termed "system B"). Base station 103 forms service area 104 and can select, from the same frequency band that is allocated to system A, channels that do not give rise to mutual interference problems, and can then re-use these channels.
FIG. 2 serves to explain mutual interference. Mutual interference has to be taken into account when base station 103 selects channels. There are four kinds of mutual interference relating to base station 101 of system A, mobile station 105 of system A, base station 103 of system B and mobile station 106 of system B. Namely:
(a) uplink interference from mobile station 106 of system B, affecting base station 101 of system A; PA1 (b) downlink interference from base station 103 of system B, affecting mobile station 105 of system A; PA1 (c) uplink interference from mobile station 105 of system A, affecting base station 103 of system B; PA1 (d) downlink interference from base station 101 of system A, affecting mobile station 106 of system B.
The four kinds of mutual interference noted above have to be taken into account in order to select channels that do not give rise to mutual interference problems. Hitherto, however, the following simplified method has been used. Namely, field monitors are disposed in the service area of system B and these measure the level of downlink interference from base stations 101 of system A on mobile station 106 of system B, i.e., the interference level in case (d) above. According to this simplified method, which is illustrated in FIG. 3, a plurality of field monitors 107 are set up in service area 104 of system B, these field monitors 107 being capable of measuring the received level from base stations 101 of system A. On the basis of instructions from base station 103 of system B, these field monitors 107 measure the received level at all frequencies (channels) used by system A and report the results of these measurements to base station 103. Having received these reports from field monitors 107, base station 103 takes into account differences in transmitted power, differences in feeder loss, etc. between the uplink and the downlink, and estimates, from the measured downlink levels, the interference level in case (a) above, i.e., the level of uplink interference from mobile station 106 of system B on base stations 101 of system A. Then, on the basis of this result, base station 103 selects channels that can be used.
FIG. 4 shows a detailed configuration of base station 103 and field monitor 107. Base station 103 has transceiver 111 for communicating with mobile stations, and base station controller 112. Field monitor 107 has field monitoring receiver 113 and received level detector 114 for measuring the received level from base stations of system A, plus controller 115 which decides from the received levels which channels can be used. Controller 115 measures the received level at all frequencies (channels) used by system A, and from these values estimates the levels of uplink interference on base stations of system A. It then compares these with a predetermined level threshold, and if an estimated interference level does not exceed a threshold, controller 115 decides that it does not cause interference at a corresponding base station of system A. Controller 115 then notifies base station controller 112 that it can use that channel. Base station controller 112 sets the channel to be used by transceiver 111 to the one notified by controller 115.
However, a drawback of this method is that in order to measure the downlink interference level accurately, it is necessary to deploy a large number of field monitors so as to ensure complete coverage of the area, with the result that setup costs are very high. Restricting the number of field monitors by deploying them at essential locations only, e.g., within the base station, has been considered, but the drawback here is that, conversely, it becomes difficult to select suitable deployment positions.
The object of the present invention is to provide a mobile communication system which overcomes this problem and significantly increases the accuracy of interference level measurement, without the need for special field monitors for measuring the level of downlink interference from the base stations of an existing system.